In a mobile communication system, a user equipment (UE) receives information from a base station (BS) in downlink (DL) and transmits information to the BS in uplink (UL). Information transmitted and received between the BS and the UE includes data and a variety of control information and various physical channels are present according to the kind/usage of the transmitted and received information.
FIG. 1 is a view showing physical channels used for a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system which is an example of a mobile communication system and a general signal transmission method using the same.
When a UE is powered on or when the UE newly enters a cell, the UE performs an initial cell search operation such as synchronization with a BS in step S101. For the initial cell search operation, the UE may receive a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the BS so as to perform synchronization with the BS, and acquire information such as a cell ID. Thereafter, the UE may receive a physical broadcast channel from the BS and acquire broadcast information in the cell. Meanwhile, the UE may receive a Downlink Reference signal (DL RS) in the initial cell search step and confirm a downlink channel state.
The UE which has completed the initial cell search may receive a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) corresponding to the PDCCH, and acquire more detailed system information in step S102.
Thereafter, the UE which has not completed access to the BS may perform a random access procedure in steps S103 to S106, in order to complete the access to the eNB. For the random access procedure, the UE may transmit a specific sequence via a Physical Random Access Channel (PRACH) as a preamble (S103), and may receive a message in response to the random access via the PDCCH and the PDSCH corresponding thereto (S104). In contention-based random access excluding handover, a contention resolution procedure including the transmission of an additional PRACH (S105) and the reception of the PDCCH and the PDSCH corresponding thereto (S106) may be performed.
The UE which has performed the above-described procedure may then receive the PDCCH/PDSCH (S107) and transmit a Physical Uplink Shared Channel (PUSCH)/Physical Uplink Control Channel (PUCCH) (S108), as a general uplink/downlink signal transmission procedure.
FIG. 2 is a view explaining a signal processing procedure of transmitting an uplink (UL) signal at a UE.
In order to transmit the UL signal, a scrambling module 201 of the UE may scramble a transmitted signal using a UE-specific scrambling signal. The scrambled signal is input to a modulation mapper 202 so as to be modulated into complex symbols by a Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) or 16-Quadrature amplitude modulation (QAM) according to the kind of the transmitted signal and/or the channel state. Thereafter, the modulated complex symbols are processed by a transform precoder 203 and are input to a resource element mapper 204. The resource element mapper 204 may map the complex symbols to time-frequency resource elements. The processed signal may be transmitted to the BS via an SC-FDMA signal generator 205 and an antenna.
FIG. 3 is a diagram explaining a signal processing procedure of transmitting a downlink (DL) signal at a BS.
In a 3GPP LTE system, the BS may transmit one or more codewords in downlink. Accordingly, one or more codewords may be processed to configure complex symbols by scrambling modules 301 and modulation mappers 302, similar to the UL transmission of FIG. 2. Thereafter, the complex symbols are mapped to a plurality of layers by a layer mapper 303, and each layer may be multiplied by a precoding matrix selected according to the channel state by a precoding module 304 and may be allocated to each transmission antenna. The processed signals which will respectively be transmitted via antennas may be mapped to time-frequency resource elements to be used for transmission by resource element mappers 305, and may respectively be transmitted via OFDM signal generators 306 and antennas.
In a mobile communication system, in a case where a UE transmits a signal in uplink, a Peak-to-Average Ratio (PAPR) may be more problematic than the case where a BS transmits a signal in downlink. Accordingly, as described above with reference to FIGS. 2 and 3, an OFDMA scheme is used to transmit a downlink signal, while an SC-FDMA scheme is used to transmit an uplink signal.
FIG. 4 is a diagram explaining an SC-FDMA scheme for transmitting an uplink signal and an OFDMA scheme for transmitting a downlink signal in a mobile communication system.
A UE for UL signal transmission and a BS for DL signal transmission are identical in that a serial-to-parallel converter 401, a subcarrier mapper 403, an M-point Inverse Discrete Fourier Transform (IDFT) module 404 and a Cyclic Prefix (CP) attachment module 406 are included.
The UE for transmitting a signal using an SC-FDMA scheme further includes a parallel-to-serial converter 405 and an N-point DFT module 402. The N-point DFT module 402 partially offsets an IDFT process influence of the M-point IDFT module 404 such that the transmitted signal has a single carrier property.
FIG. 5 is a diagram explaining a signal mapping scheme in a frequency domain satisfying a single carrier property in the frequency domain. FIG. 5(a) shows a localized mapping scheme and FIG. 5(b) shows a distributed mapping scheme. Currently, the localized mapping scheme is defined in a 3GPP LTE system.
A clustered SC-FDMA scheme which is a modified form of the SC-FDMA scheme will now be described. In the clustered SC-FDMA scheme, DFT process output samples are sequentially divided into sub-groups and are mapped to subcarrier regions which are separated from each other on a per sub-group basis in an IFFT sample input unit in a subcarrier mapping process between a DFT process and an IFFT process. In some cases, a filtering process and a cyclic extension process may be included.
At this time, a sub-group may be called a cluster and cyclic extension means that a guard interval longer than maximum delay spread of a channel is inserted between contiguous symbols in order to prevent inter-symbol interference (ISI) while each subcarrier symbol is transmitted via a multi-path channel.